In general, when a railroad car enters into a railroad tunnel, a pressure wave is formed. Such a pressure wave is propagated into the tunnel to be emitted outward in a micro pressure wave form through a tunnel exit. Since the micro pressure wave causes impulsive noise and low frequency vibration to peripheral private houses, it is highly important to reduce the micro pressure wave in designing a railroad tunnel.
Accordingly, for the reduction of the micro pressure wave, a hood having an arch-shaped cross section has been conventionally provided at an entry of a railroad tunnel. Such a hood has been significantly effective in reducing the micro pressure wave.
However, with the recent tendency of increase in the traveling speed of railroad cars and length of railroad tunnels, the tunnel micro pressure wave has been further increased. Accordingly, in order to reduce the micro pressure wave while using the conventional technology that provides a hood at an entry of a railroad tunnel, inner hole cross-sectional area should be widened and a length of the hood should be lengthened. However, there have been found the problems set forth below.
Since a railroad has limit in a roadbed width, and many facilities such as pillars for wiring devices are provided at an entry of a tunnel, there have been significant difficulties in enlarging or lengthening a hood. Further, if a length and an inner hole cross-sectional area of the hood increase, a thickness and hardness, etc., of the hood should also be further increased in order to assure structural stability and others, resulting in increase of construction costs. As a result of these problems, the conventional methods for lengthening a hood or widening a hood inner hole cross-sectional area have had limit in reducing the micro pressure wave.
Further, in recent, there has been a continuous tendency to reduce the tunnel inner hole cross-sectional area in order to reduce construction costs for construction of a 180 km/h or more high speed railroad, and concrete slab tracks have been used, instead of pebble ballast tracks, in order to reduce maintenance and management costs for tracks. However, impulsive noise/vibration by the micro pressure wave of a tunnel exit significantly increases in the small sectional tunnel having slab tracks. Thus, measures to effectively greatly reduce the impulsive noise/vibration are necessary.